Polyethylene glycol (PEG) is widely known as a standard carrier in the drug delivery system and the development is being vigorously advanced even now. By modifying a physiologically active substance or a drug, for example, a liposome with polyethylene glycol, due to the high hydration layer and steric repulsion effect of polyethylene glycol, capture by the reticuloendothelial system (RES) or excretion in the kidney of the drug is suppressed and it is possible to improve the circulation in blood and to decrease the antigenicity. Among them, polyethylene glycol having a carboxyl group at the terminal is a particularly important material because it is used as a raw material for converting into an activated ester to form a stable amide bond with a lysine residue or a primary amino group at the N-terminal of a protein or other active groups. As a PEGylated preparation using polyethylene glycol having a carboxyl group as a raw material, SOMAVERT (registered trademark): PEGVISOMANT (Pfizer), Mircera (registered trademark): Methoxy Polyethylene glycol-epoetin beta (F Hoffmann-La Roche), Macugen (registered trademark): Pegaptanib (Eyetech) and the like are being placed on the market. In addition, clinical trials of many other PEGylated preparations are conducted, so that its utility is demonstrated.
As to the activated polyethylene glycol for the purpose of the medical applications, it has been demanded to reduce impurities from the standpoint of performance and safety of a drug produced by modifying the same. At present, as to the polyethylene glycol compound having a carboxyl group, those having various backbones and linkers between a carboxyl group and a polyethylene glycol have been developed and various impurities are by-produced according to the production methods thereof. Examples thereof are shown below.
As to a production method of a straight-chain polyethylene glycol having a carboxyl group, there have been reported a method of oxidizing a terminal hydroxyl group of polyethylene glycol with a heavy metal reagent described in Non Patent Literature 1, a method in which a terminal hydroxyl group of polyethylene glycol and an acrylic acid ester compound are bonded by Michael reaction and then ester hydrolysis is performed described in Patent Literature 1, a method in which a terminal hydroxyl group of polyethylene glycol and an ortho ester compound having a leaving group are bonded by Williamson reaction and then hydrolysis is performed described in Patent Literature 2, and the like.
Further, as to a branched polyethylene glycol having one carboxyl group, a method in which methoxypolyethylene glycol is introduced into two amino groups of lysine ethyl ester and then hydrolysis of the ester is performed and the like is reported in Patent Literature 3.
However, the conversion rate to a carboxyl group at the terminal of polyethylene glycol is not high in any of the production methods described above, and in the highest a carboxylic acid purity is around 90%. In the case where a drug is modified in this state, many impurities, for example, an unreacted drug or an unreacted or deactivated polyethylene glycol compound are contained in addition to the intended PEGylated preparation. Purification of the PEGylated preparation causes a technical problem in that separation is difficult and a problem in that the production cost is increased due to the decrease in drug yield. Thus, it is desirable to remove the polyethylene glycol compounds as impurities as much as possible before the bonding to the drug.
In the case where impurities, for example, a low-molecular compound or a salt are removed from a polymer compound, for example, polyethylene glycol, purification by crystallization and extraction operation utilizing physicochemical properties caused by a molecular weight, in particular, a large difference in solubility to a solvent is commonly performed. On the other hand, in the case where the polyethylene glycol compound as an impurity is present, a large difference in the physicochemical properties, for example, solubility, between the impurity and the polyethylene glycol compound of the objective substance does not appear. Therefore, it is difficult to remove the polyethylene glycol compound having no carboxyl group from the polyethylene glycol compound having one carboxyl group by a conventional operation for removing low-molecular impurities and to perform purification.
As to a purification method of a polyethylene glycol compound having a carboxyl group, column purification by an ion-exchange resin is performed by using the polarity thereof, and purification of a branched polyethylene glycol is described in Patent Literature 4. However, the concentration of the polyethylene glycol compound in the process must be in a high dilution condition of approximately from 0.1 to 2% in order to suppress lowering of the separation ability, and a large-capacity extraction facility or a concentration facility for recovering the polyethylene glycol compound in the aqueous solution is required after the fractionation, so that industrial productivity cannot be sufficiently satisfied. Moreover, since a large amount of the ion-exchange resin is finally discharged as a waste, the purification method has an industrial problem.
Further, a method of purification in which polyethylene glycol having a carboxyl group dissolved in an organic solvent is physically adsorbed to inorganic adsorbent and then desorbed using an alcoholic solvent is described in Patent Literature 5. However, in the purification method, the desorption reaction is insufficient and the yield is low as from 50 to 60%. Moreover, since a large amount of the adsorbent is finally discharged as a waste, the purification method has an industrial problem.
On the other hand, a method of purification in which a polyethylene glycol compound having an amino group is dissolved in a strongly acidic aqueous solution having a pH of 1 to 3 to ionize a terminal amino group and extracted in a specific temperature range using a specific mixed organic solvent is described in Patent Literature 6. According to the patent, the polyethylene glycol compound having an amino group whose hydrophilicity is increased by the ionization is distributed into an aqueous layer and a polyethylene glycol compound having no amino group is distributed into a mixed organic layer, so that selective separation and purification can be performed.